Polyethylene Crystals Research Articles

Dynamic Modeling of Ship-to-Ship and Ship-to-Pier Mooring Performance

AbstractTraditional methods for the design of ship-to-pier moorings in normal and storm conditions and for ship-to-ship moorings under normal conditions are currently based on static calculations. These calculations have served well for many years, first with natural fiber ropes and later with nylon and other low- to medium-modulus synthetics. Key to the success of this simplistic approach is lines that can elongate enough under tension to share the loads between multiple lines. When wire rope mooring lines are introduced, an increased weight catenary and the use of constant tension winches allowed enough compliance for the moorings to load share successfully.Now, we have very lightweight, high-modulus synthetic lines like High Modulus Polyethylene (HMPE), Aramid, and Liquid Crystal Polymer (LCP), where there is almost no stretch and very little weight to form a weight catenary. When used with constant tension winches that allow the mooring load to be shared across multiple lines, these can work well. However, when they are used from bollard to chock to bit with no compliance, they are unable to share the load between multiple lines, and high tension failures occur where a weaker but more compliant mooring line would be fine.This article describes advanced dynamic modeling of ships loaded by wind, waves, and currents in these conditions and the tension sharing between mooring lines of different materials and constructions. The need to share the mooring load between multiple lines is the crux of the issue.

Shear induced crystallization of bimodal and unimodal high density polyethylene

In-situ step-shear experiments by synchrotron X-ray techniques were performed to study shear-induced crystallization of bimodal (BM1) and unimodal (UM1) high-density polyethylene (HDPE). The BM1 sample consisted of a bimodal molecular weight (MW) and composition distribution with the low MW components having less short chain branches (SCBs), or hexene co-monomer, and the high MW components having more SCBs. The average molecular weight of BM1 was higher than that of UM1, where both BM1 and UM1 covered the same molecular weight range. The UM1 sample contained a unimodal but broad molecular weight distribution (MWD) with a constant SCB value between those of the low and high MW components in BM1. Discrete primary and secondary lamellae were found in BM1 and UM1, whereas UM1 exhibited faster secondary lamellar growth. Shear-induced crystallization resulted in higher lamellar orientation in UM1, but higher crystallinity in BM1. The large fraction of medium high MW components in UM1 was found to be effective in crystallization under deformation, leading to stronger shear-induced crystalline orientation.

Structure of the in situ produced polyethylene based composites modified with multi-walled carbon nanotubes: In situ synchrotron X-ray diffraction and differential scanning calorimetry study

Polyethylene based composites modified with multi-walled carbon nanotubes (MWCNTs) were produced via in situ polymerization of ethylene with the Ti-Ziegler–Natta catalyst preliminarily immobilized on MWCNTs. The composite structure was characterized with transmission and scanning electron microscopy, differential scanning calorimetry (DSC) and in situ synchrotron X-ray Diffraction (in situ XRD). For the first time the Ti-containing catalyst species of the size 2–3 nm were observed on the MWСNTs surface stabilized in the polymer matrix. A comparative study of the melting-crystallization cycles of neat polyethylene (PE) and MWCNT-PE composites with in situ XRD and DSC provide information on the nucleation of PE crystals. For the first time, the in situ XRD technique was used for estimation of the coherent scattering region of PE blocks during the melting-crystallization cycles. These experiments and molecular dynamic modeling showed that MWCNTs act as the template for the PE chain orientation and as the nucleating agent for PE crystallization. However, the nucleation of PE crystals in composites occurs on the nanotube surface and also within the space between nanotubes. Thus, the relative volume of PE nucleated on nanotubes depends on their content in the composite and can be significant only for the composites with high nanotube loading.

Synthesis of polyethylene/silica-silver nanocomposites with antibacterial properties by in situ polymerization

Synthesis of polyethylene/silica-silver nanocomposites (PE/SiAg) by in situ polymerization with supported and non-supported catalysts was achieved using the Cp2ZrCl2/MAO catalytic system. Silica-silver nanoparticles (SiAg) were synthesized via two routes (acidic and basic) and characterized to determine the silver content, morphology, and size. The basic route resulted in particles with a lower concentration of silver and with smaller diameters. The polymerizations of ethylene in the presence of the fillers produced high yields of nanocomposites. The catalyst support in SiAg was efficient, although the percentage of Zr effectively immobilized was very low. Polyethylene melting and crystallization temperatures did not change significantly with the addition of the filler. SEM images showed differences in the morphologies between the supported and non-supported catalysis, and between the acidic and basic conditions for SiAg preparation. Two different tests were performed and showed that the nanocomposites inhibited the proliferation of bacteria in contact with the films.

Flow induced crystallization prevents melt fracture of HDPE in uniaxial extensional flow

This work concerns extension induced crystallization of a commercial high density polyethylene above the equilibrium melting temperature. We compare the nonlinear response during uniaxial elongation to the morphology obtained in the quenched fibers after cessation of the flow at a Hencky strain of 5. At 12 °C above the melting temperature, the samples undergo brittle fracture. Samples stretched at 2 and 6 °C above the melting temperature remain intact throughout the entire course of deformation and exhibit a strain hardening behavior that does not follow time temperature superposition. We propose that stabilization of the filament at lower temperatures, as well as the failure of time temperature superposition, is caused by flow-induced nucleation and growth of shish structures oriented along the flow direction. Further justification is obtained from small-angle X-ray scattering performed on the quenched filament showing an increased formation of shish with an increase in the deformation rate. We find the critical Hencky strain for the onset of the shish formation to be between 0 and 0.6, which is significantly lower than the values reported in the existing literature. We model the influence of shish nucleation on the rheological response in an extension using the hierarchical multimode stress function, which is modified to include the stretched network assumption.This work concerns extension induced crystallization of a commercial high density polyethylene above the equilibrium melting temperature. We compare the nonlinear response during uniaxial elongation to the morphology obtained in the quenched fibers after cessation of the flow at a Hencky strain of 5. At 12 °C above the melting temperature, the samples undergo brittle fracture. Samples stretched at 2 and 6 °C above the melting temperature remain intact throughout the entire course of deformation and exhibit a strain hardening behavior that does not follow time temperature superposition. We propose that stabilization of the filament at lower temperatures, as well as the failure of time temperature superposition, is caused by flow-induced nucleation and growth of shish structures oriented along the flow direction. Further justification is obtained from small-angle X-ray scattering performed on the quenched filament showing an increased formation of shish with an increase in the deformation rate. We find the ...

High-density polyethylene crystals with double melting peaks induced by ultra-high-molecular-weight polyethylene fibre.

High-density polyethylene (HDPE)/ultra-high-molecular-weight polyethylene (UHMWPE) fibre composites were prepared via solution crystallization to investigate the components of epitaxial crystal growth on a highly oriented substrate. Scanning electron microscopy morphologies of HDPE crystals on UHMWPE fibres revealed that the edge-on ribbon pattern crystals that were formed initially on UHMWPE fibres converted afterwards to a sheet shape as crystallization progressed. Wide-angle X-ray diffraction confirmed that the polymer chain oriented along the fibre axis and the orthorhombic crystal form of HDPE remained unchanged in HDPE/UHMWPE fibre composite systems. The thermal behaviour of the fibre composites measured by differential scanning calorimetry showed double melting peaks, the nature of which, as disclosed by partial melting experiments, is ascribed to bilayer components existing in the induced crystals: the inner layer is composed of more regularly folded chain crystals induced by UHMWPE fibres, and the outer layer formed on the inner one with a thinner and lower ordered crystal structure.

Crystal Growth in Polyethylene by Molecular Dynamics: The Crystal Edge and Lamellar Thickness.

We carried out large-scale atomistic molecular dynamics simulations to study the growth of twin lamellar crystals of polyethylene initiated by small crystal seeds. By examining the size distribution of the stems—straight crystalline polymer segments—we show that the crystal edge has a parabolic profile. At the growth front, there is a layer of stems too short to be stable, and new stable stems are formed within this layer, leading to crystal growth. Away from the edge, the lengthening of the stems is limited by a lack of available slack length in the chains. This frustration can be relieved by mobile crystal defects that allow topological relaxation by traversing through the crystal. The results shed light on the process of polymer crystal growth and help explain initial thickness selection and lamellar thickening.

Direct All-Atom Molecular Dynamics Simulations of the Effects of Short Chain Branching on Polyethylene Oligomer Crystal Nucleation

Using all-atom molecular dynamics (MD) simulations, we demonstrate that short alkyl branches can hinder the nucleation of polyethylene (PE) oligomers. Although one methyl and ethyl branch in a 50-carbon oligomer may only slow the nucleation kinetics mildly, bulkier side chains, such as propyl, butyl, and hexyl branches, disturb the arrangement of neighboring backbone atoms, preventing these atoms from joining a growing crystal, and therefore significantly suppress the nucleation of PE crystals, with no clear evidence of nucleation being observed over a 20 ns simulation run when hexyl side chains are present. The degrees of branching and the distributions of short alkyl groups on PE backbones can also affect the crystallization kinetics, with well-spaced branches having a greater impact on crystallization than branches that are grouped within a shorter distance along the backbone that is similar to or shorter than the length of the branch. We show that the linear portions of PE crystallize first and the br...

Superheated Melting Kinetics of Metastable Chain-Folded Polymer Crystals

The unique behavior of superheated melting kinetics of polymer crystals has been examined in terms of the metastable nature of thin polymer crystals with chain folding. The superheated melting kinetics was characterized by the heating rate dependence of the melting peak in the thermogram. By examining the behaviors of polyethylene molar mass fractions, its homologue, hexacontane, and indium, it has been experimentally confirmed that the metastability of crystals with chain folding has an essential role in the superheated melting kinetics; i.e., stable extended-chain crystals of hexacontane melts in the same way as indium without superheating, and metastable chain-folded crystals of higher molar mass polyethylene needs to overcome a larger kinetic barrier for melting.

Influences of the chain structure of PE‐b‐PEG on the properties of PE/PE‐b‐PEG blend membranes prepared by TIPS

ABSTRACTIn this article, a series of diblock copolymer polyethylene‐b‐ poly(ethylene glycol)s (PE‐b‐PEGs) with various molecular weight of polyethylene segment was blended with linear low‐density PE. The PE/PE‐b‐PEG blend porous membranes with high porosity were obtained by thermally induced phase separation (TIPS) process. The isothermal crystallization kinetics of PE/LP/PE‐b‐PEG blends indicated that the introduction of PE‐b‐PEG could inhibit the growth rate of polyethylene crystals which could increase the pore size and porosity of the membranes. The PE/PE‐b‐PEG blend membranes with PE1300‐b‐PEG2200 showed the largest pore size and porosity due to its crystallization behavior during TIPS. The surface of the membranes became smoother and the morphology of the membranes could be effectively tuned by introducing PE‐b‐PEG. Compared with the PE membrane, the PE/PE‐b‐PEG blend membranes exhibited higher hydrophilicity (the water contact angle decreased from 112° to 84°), water permeability (the permeation flux increased from 80 to 440 L/m2 h under 0.1 MPa), rejection performance (completely reject carbon particles in the filtration of carbon ink solution), and fouling resistance (the value of protein adsorption dropped from 0.25 to 0.05 mg/cm2). The hydrophilicity and fouling resistance of PE/PE‐b‐PEG blend membranes increased as the length of PE segment in PE‐b‐PEGs decreased. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46499.

The influence of short chain branch on formation of shear induced crystals in bimodal polyethylene at high shear temperatures

The shear-induced crystallization of bimodal polyethylene with high molecular weight fraction having different content of short chain branch at low shear rate and high temperatures was investigated by in-situ small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. SAXS and WAXD results indicate that shish crystals form at different shear temperatures of 139 and 136 °C, subsequently kebab crystals and oriented lamellae form at the isothermal crystallization temperature of 121 °C. The presence of short chain branch significantly promotes the formation of shish-kebab crystals owing to the increase of the relaxation time, but overmuch short chain branch impedes the formation of shish-kebab crystals and form more oriented lamellae. These evidences clearly indicate that even though the existence of short chain branch impedes the crystallizability, it can still effectively promote the formation of shish-kebab crystals at high shear temperatures.

The influence of short chain branch on formation of shish‐kebab crystals in bimodal polyethylene under shear at high temperatures

ABSTRACTFormation of shish‐kebab crystals due to the coil–stretch transition under shear in the molten state using a bimodal polyethylene system with high molecular weight (HMW) fraction having different branch content was investigated. In specific, in situ small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray diffraction (WAXD) techniques were used to study the structure evolution of shish‐kebab crystals at high temperatures under simple shear. The SAXS results revealed that with the increase of branch content, shish‐kebab crystals became more stable at high temperatures (e.g., 139 °C). However, the shish length of the bimodal PE containing 0.11% branch was shorter than that with no branch. The WAXD results showed that the degree of crystallization for bimodal PE with HMW fraction having 0.11% branch increased with time but reached a plateau value of 1%, while that with no branch increased continuously till 11%. Furthermore, the crystal orientation of bimodal PE with HMW fraction having 0.11% branch was above 0.9 and maintained at a constant value, while that with no branch decreased from 0.9 to 0.1 upon relaxation. This study indicates that even though the crystallizability of the HMW fraction with branch content decreased, they could effectively stabilize the shear‐induced crystalline structure with shorter shish‐kebab crystals. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 786–794

The role of a secondary process in polymer crystallization: 4 − Polyethylene revisited

AbstractIt is suggested that the overlap of primary and secondary crystallization is a cause of the repeated observation of non‐integer n values in polymer crystallization. The possibility that two concurrent crystallization processes occurring during the crystallization of polyethylene may account for anomalous fractional values of the Avrami exponent, of no theoretical significance, is reconsidered using data from the classical paper by Banks et al. (Polymer 4: 61 − 74 (1963)) on the kinetics of crystallization of polyethylene fractions which placed emphasis on evaluating the secondary crystallization stage. In general constant n values in excess of that expected for the crystallization mechanisms can readily be interpreted in terms of the additional crystallinity developed by the secondary process and these values increase commensurate with the rate constant of secondary crystallization. Constant n values close to but below n = 4.0 are interpreted as due to the presence of an induction period present in the presence of homogeneous nucleation. It is not possible to separate the effect of incorporating both an induction period, lowering the value of n, and secondary crystallization, raising the value of n, in the kinetic equation. © 2017 Society of Chemical Industry

DFT Modeling of Cross-Linked Polyethylene: Role of Gold Atoms and Dispersion Interactions

Using DFT modeling, we analyze the concerted action of gold atoms and dispersion interactions in cross-linked polyethylene. Our model consists of two oligomer chains (PEn) with 7, 11, 15, 19, or 23 carbon atoms in each oligomer cross-linked with one to three Au atoms through C-Au-C bonds. In structures with a single gold atom the C-Au-C bond is located in the central position of the oligomer. Binding energies (BEs) with respect to two oligomer radical fragments and Au are as high as 362-489 kJ/mol depending on the length of the oligomer chain. When the dispersion contribution in PEn-Au-PEn oligomers is omitted, BE is almost independent of the number of carbon atoms, lying between 293 and 296 kJ/mol. The dispersion energy contributions to BEs in PEn-Au-PEn rise nearly linearly with the number of carbon atoms in the PEn chain. The carbon-carbon distance in the C-Au-C moiety is around 4.1 Å, similar to the bond distance between saturated closed shell chains in the polyethylene crystal. BEs of pure saturated closed shell PEn-PEn oligomers are 51-187 kJ/mol. Both Au atoms and dispersion interactions contribute considerably to the creation of nearly parallel chains of oligomers with reasonably high binding energies.

Polyethylene nano crystalsomes formed at a curved liquid/liquid interface.

Crystallization is incommensurate with nanoscale curved space due to the lack of three dimensional translational symmetry of the latter. Herein, we report the formation of single-crystal-like, nanosized polyethylene (PE) capsules using a miniemulsion solution crystallization method. The miniemulsion was formed at elevated temperatures using PE organic solution as the oil phase and sodium dodecyl sulfate as the surfactant. Subsequently, cooling the system stepwisely for controlled crystallization led to the formation of hollow, nanosized PE crystalline capsules, which are named as crystalsomes since they mimic the classical self-assembled structures such as liposome, polymersome and colloidosome. We show that the formation of the nanosized PE crystalsomes is driven by controlled crystallization at the curved liquid/liquid interface of the miniemulson droplet. The morphology, structure and mechanical properties of the PE crystalsomes were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and atomic force spectroscopy. Electron diffraction showed the single-crystal-like nature of the crystalsomes. The incommensurateness between the nanocurved interface and the crystalline packing led to reduced crystallinity and crystallite size of the PE crystalsome, as observed from the X-ray diffraction measurements. Moreover, directly quenching the emulsion below the spinodal line led to the formation of hierarchical porous PE crystalsomes due to the coupling of the PE crystallization and liquid/liquid phase separation. We anticipate that this unique crystalsome represents a new type of nanostructure that might be used as nanodrug carriers and ultrasound contrast agents.

Local structure order assisted two-step crystal nucleation in polyethylene

Homogeneous nucleation process of polyethylene (PE) is studied with full-atom molecular dynamic simulation. To account the complex shape with low symmetry and the peculiar intra-chain conformational order of polymer, we introduce a shape descriptor OCB coupling conformational order and inter-chain rotational symmetry, which is able to differentiate hexagonal and orthorhombic clusters from melt. With the shape descriptor OCB, we find that coupling between conformational and inter-chain rotational orderings results in the formation of hexagonal clusters first, which is dynamic in nature. Whilst nucleation of orthorhombic structure occurs inside of hexagonal clusters later, which proceeds via the coalescence of neighboring hexagonal clusters rather than standard stepwise growth process. This demonstrates that nucleation of PE crystal is a two-step process with the assistance of OCB order, which is different from early models for polymer crystallization but similar with that proposed for spherical 'atoms' like colloid and metal.

Vibrational Analysis of Semicrystalline Polyethylene Using Molecular Dynamics Simulation

The vibrational spectra of semicrystalline polyethylene and its distinct domains were investigated using molecular dynamics (MD) simulations. A method for the vibrational analysis of the domains within the lamellar stack model of semicrystalline polymers has been developed and demonstrated on semicrystalline polyethylene using force fields having either united atom (UA) or explicit atom (EA) detail. In the UA description, the calculated vibrational spectra were found to differ from the observed skeletal vibrations of polyethylene with the force field used in this work. Therefore, a modified UA force field with different stretching and bending force constants is proposed, which was found to reproduce the observed frequencies of the skeletal vibrations. In the EA description, the vibrational spectra of semicrystalline polyethylene were in satisfactory agreement with typical infrared and Raman signatures of polyethylene melts and crystals. Experimental interpretations regarding the assignment of peaks in the Raman spectra to components of semicrystalline polyethylene were examined. The spectrum of the interphase domain obtained using the EA model was found to be adequately reproduced by a superposition of the spectra of the crystalline and amorphous domains, at variance with experimental observation. The lack of a distinct interphase spectrum in the EA model was attributed to the absence of the CH2 bending peak associated with the orthorhombic phase, despite confirming an orthorhombic crystal structure in the crystalline domain.

Three-Dimensional Printed Shape Memory Objects Based on an Olefin Ionomer of Zinc-Neutralized Poly(ethylene-co-methacrylic acid).

Three-dimensional printing enables the net shape manufacturing of objects with minimal material waste and low tooling costs, but the functionality is generally limited by available materials, especially for extrusion-based printing, such as fused deposition modeling (FDM). Here, we demonstrate shape memory behavior of 3D printed objects with FDM using a commercially available olefin ionomer, Surlyn 9520, which is zinc-neutralized poly(ethylene-co-methacrylic acid). The initial fixity for 3D printed and compression-molded samples was similar, but the initial recovery was much lower for the 3D printed sample (R = 58%) than that for the compression-molded sample (R = 83%). The poor recovery in the first cycle is attributed to polyethylene crystals formed during programming that act to resist the permanent network recovery. This effect is magnified in the 3D printed part due to the higher strain (lower modulus in the 3D printed part) at a fixed programming stress. The fixity and recovery in subsequent shape memory cycles are greater for the 3D printed part than for the compression-molded part. Moreover, the programmed strain can be systematically modulated by inclusion of porosity in the printed part without adversely impacting the fixity or recovery. These characteristics enable the direct formation of complex shapes of thermoplastic shape memory polymers that can be recovered in three dimensions with the appropriate trigger, such as heat, through the use of FDM as a 3D printing technology.

Insight into understanding the evolution of the epitaxy crystallization in isotactic polypropylene and polyethylene blends

In this article, epitaxial structures have been successfully obtained in the isotactic polypropylene (iPP)/polyethylene (PE) blends by an accessible injection molding methods. By studying a series of iPP/PE blends, the evolution of the epitaxial growth of PE lamellae on the oriented iPP lamellae has been detailedly discussed via wide‐angle X‐ray diffraction, small‐angle X‐ray scattering, scanning electron microscopy and differential scanning calorimetry. Unexpectedly, the exactly epitaxial angles between peculiarly arranged PE lamellae and oriented PP lamellae are all larger than the classical epitaxy theory value of 50°, and it even increases gradually with increasing PP content. It is inferred that the special crystallization of PE is the consequence of joint construction of the oriented PP crystals and the continuous intense shear field provided by pressure vibration injection molding. The epitaxial structures play a positive role in the interfacial connection between two components; thus, the mechanical properties of the blends are improved. This work provides an insight understanding on the formation mechanism of the epitaxy crystallization under shear field. Copyright © 2017 John Wiley & Sons, Ltd.

Coupling between intra- and inter-chain orderings in flow-induced crystallization of polyethylene: A non-equilibrium molecular dynamics simulation study.

Non-equilibrium molecular dynamics simulations have been performed to study the molecular mechanism of flow-induced crystallization (FIC) of polyethylene (PE). The end-to-end distance of chain Rete and the content of trans conformation Ctrans are extracted out to represent intra-chain conformation ordering at whole chain and segment levels, respectively, while orientation correlation function P, density ρ, and bond orientational order parameter Q4 are taken to depict inter-chain orders. Imposing the extension induces the intra-chain conformational ordering to occur first, which further couples with the inter-chain order and results in the formation of hexagonal packing. Further increasing strain leads to the appearance of orthorhombic order. The results demonstrate that the FIC of PE proceeds via a multi-stage ordering process, during which coupling occurs among stress, intra-chain conformation, and inter-chain orientation and density orderings. Analyzing the flow-induced energy evolution unveils that not only entropy but also energy plays an important role in the FIC.